A spark plug is mounted to an internal combustion engine and used to ignite air-fuel mixture in a combustion chamber. Generally, a spark plug includes an insulator having an axial hole extending in the direction of an axis, a center electrode inserted into the axial hole, and a metallic shell provided externally of the outer circumference of the insulator. The metallic shell has, on its outer circumferential surface, a threaded portion that is dimensioned to threadingly engage with a mounting hole of a head of the internal combustion engine. A screw neck extends rearward from the rear end of the threaded portion. A diameter-expanded portion is located rearward of the screw neck and has a diameter greater than that of the screw neck. A seat portion connectingly extends between the screw neck and the diameter-expanded portion. Additionally, a ring-like gasket is provided around the screw neck in contact with the seat portion. When the spark plug is mounted to the internal combustion engine, an axial force associated with screw engagement brings the gasket into close contact with the head of the internal combustion engine, thereby maintaining gas tightness (See, for example, Japanese Patent Application Laid-Open (kokai) No. 2008-108478).
In view of implementation of further improved gastightness, bringing the seat portion and the head directly into close contact with each other without provision of the gasket is conceived (See, for example, Japanese Patent Application Laid-Open (kokai) No. 2011-118659).
However, spark plugs of such a type may encounter impairment in gastightness caused by occurrence of slight damage, strain, or the like on the seat or the head.
In recent years, in order to improve layout flexibility for an engine head (or for a like purpose) a reduction in the size (diameter) of a spark plug is required, leading to a reduction in the diameter of the diameter-expanded portion and the threaded portion of the metallic shell. A reduction in the diameter of the diameter-expanded portion inevitably leads to a reduction in the area of the seat portion. Also, a reduction in the diameter of the threaded portion may lead to a reduction in an axial force associated with screw engagement. That is, a diameter-reduced spark plug encounters difficulty in ensuring a sufficient seal between the seat portion and the head. Eventually, the gastightness of a combustion chamber is apt to be impaired.
The present invention has been conceived in view of the above circumstances, and provides a spark plug for an internal combustion engine capable of ensuring sufficient gastightness of a combustion chamber and meeting demand for a reduction in diameter.